Classic spectrophotometers can be used to determine optical properties of bacteria using absorption or scattering. Absorption spectrophotometers can be used to measure the relative absorbance of a sample. Absorbance is measured by comparing the intensity of light entering a sample with the intensity of light exiting the sample. A drop in light intensity indicates a quantity of light has been absorbed. This can be displayed as an arbitrary figure, typically an optical density. This can lead to an accurate count of the number of cells present in a sample.
Scattering spectrophotometers usually comprise an intense light source, such as a laser or a very bright incandescent source, and a monochromator. Light is incident on a sample and is scattered at different angles. Detectors placed at discrete intervals around a chamber collect the scattered light. Collected light in the side scattering region can be used to obtain information about granularity and light collected in the forward scattering region can be used to obtain information about the size of the particles. Overall intensity of the scattered light gives a turbidity reading and an indication of the number of particles present. In scattering spectrophotometers for measuring bacteria, the typical wavelength of the light source is 600 nm. This wavelength is the most scattered and least absorbed by a number of organic materials, such as DNA, proteins, cytochromes.
Flow cytometers can also determine properties of a sample of interest. When a sheath-flow of index matched liquid flows through a narrow tube, the liquid acts to reduce the lumen of the tube forcing cells in the liquid to pass through the tube individually. This facilitates cell counting. Laser light incident on the narrow tube is scattered as individual cells pass through. Side and forward scattering data can be recorded to give information about the size and granularity of the cells under study. Thousands of cells can pass through the beam and be measured in this way in a few seconds and in very little liquid. Whilst cytometers are useful in some applications, they are sophisticated machines that require extensive training of an operator. Safe operation also requires a regular input of reagents and this contributes to on-going running costs. The interpretation of data produced can also prove challenging.
Another method for measuring concentration of suspended particles in a liquid or gas is nephelometry. Nephelometers can be configured to use integrating spheres. In such a configuration, light is incident on a sample and may be scattered by particles in the sample before entering the integrating sphere. The scattered light is then reflected and diffused inside the integrating sphere before being detected at an exit port of the sphere. Unscattered light passes straight through the sphere and is not collected.